Engine manifolds heretofore have been typically formed of grey cast iron with heavy walls defining ducts or channels, each curving rearwardly from the exhaust ports of an engine to carry exhaust gas to a common pipe. However, in recent years exhaust gas temperatures have been increasing as a result of modifications to the combustion process to reduce emissions. Grey iron is affected by such temperatures, and thus strengthening ribs have been added to such manifolds contrary to another developing need for lighter weight.
The use of grey iron manifolds has been further aggravated by (a) the use in some instances of noncatalyzed secondary combustion in the exhaust manifold with or without the introduction of secondary air to facilitate additional emission cleanup, and (b) by the mounting of a catalytic converter directly on the manifold so that a dual stage emission cleanup can take place in close proximity to the source of emission. As a result, the exhaust gas temperature within the manifold is significantly elevated, which in turn demands greater high temperature strength characteristics from the manifold material.
These cummulative conditions place unusual thermal and physical requirements upon the construction of a manifold which, when coupled with the requirements for reduced weight in all engine components, cannot be met by the state of the prior art. Cast metals offer the most hope, but all lose some strength as operating temperatures increase. This strength problem is compounded if the walls of the manifold are reduced in thickness to save weight; a new structural design approach is needed to meet the above needs.